The pressure exponent (n) of a propellant is a measure of the increase in burning rate of a propellant which occurs as the chamber pressure is increased. The pressure exponent (n) is the tangent to the curve which can be drawn when the burning rate is plotted against chamber pressure. The pressure exponent would be zero for a propellant whose burning rate is totally independent of pressure.
A pressure exponent of less than 0.5 is necessary for a propellant to be acceptable for use in propulsion subsystems. The exception is pressure-sensitive propellants which are intended for use in controllable motors. Considering the smokeless NF-propellant, as an illustration, it has been necessary to resort to the use of a mixture of ammonium perchlorate (20%) and HMX (cyclotetramethylenetetranitroamine) as oxidizer to result in a propellant with a pressure exponent approaching 0.5. The replacement of a portion of the HMX oxidizer with ammonium perchlorate lowered the pressure exponent of the propellant from 0.8 to a value of 0.6. Accordingly, it can be seen that there is a need for materials that can be used in propellant compositions to lower the pressure exponent thereof.
Therefore, it is an object of this invention to provide chemical constituents that can be used in propellant compositions to lower the pressure exponent of the propellant.
Another object of this invention is to provide pressure exponent suppressants that can be substituted for ammonium perchlorate in smokeless type propellants.
A further object of this invention is to provide pressure exponent suppressants that can be added in small quantities, say from 1 to 5 percent by weight, to presently known propellant compositions to lower the pressure exponents thereof to an acceptable level for a specific use.